Cache key test is incomplete

In tests/cachekey/project/elements/build1.bst, I added a comment that it is only interesting to test the manual elements here.

But I was wrong - in a stable release cycle it is important to catch any changes to BuildElement specific defaults which may effect the cache key illegally.

This should be enhanced to ensure that we have at least one .bst file which the main tests/cachekey/project/target.bst target depends on, for each BuildElement implementation.